Try this one: https://www.bikeforums.net/folding-bikes/1163449-brompton-what-changes-over-years-innovations.html

I was in your shoes a few months ago:

Basically: 2007 or newer
Pretty much all models are upgradable.

Personally, I saw it's about $1000 USD shipped to US for my cutoff.

If you have friends in England it's cheaper potentially.

If you have friends in Asia (Japan, Singapore, etc), you should look into a Tyrell Ive. It's imho a superior bike to the Brompton... better construction, better geometry, stock parts, better ride, slightly bigger.

That being said, I love my Brompton.